Method of detecting flaws in rigid material



Jan. 9, 1940. e. E LLlS 2,186,014

METHOD OF DETECTING FLAWS IN RIGID MATERIAL Filed Nov. 7, 1938 EMEg. /U5 wee/2%.

. Patented Jan. 9, 1940 UNETED STATES PATENT OFFECE METHOD OF DETECTING FLAWS IN RIGID MATERIAL Pennsylvania Application November 7, 1938, Serial No. 239,337

16 Claims.

This invention relates to a method of treating rigid materials of relatively high elastic limit to make flaws and cracks therein, not apparent to the naked eye, capable of easy detection. More particularly, it relates to a method of making flaws and cracks therein capable of easy visual detection, by coating such articles with a continuous adherent brittle film and applying appropriate loads thereto.

It has heretofore been proposed to include in a process for measuring strains in metallic articles a preliminary step comprising the coating of such articles with a brittle film of varnish or resinous compositions, to make the direction of elastic strains in such articles capable of visual detection. Obviously, sumcient elongation of an article so coated causes the film to crack perpendicularly to the strain. A following step in the process comprises repeating the stressing of the article and measuring the elongation produced whose direction has previously been determined, by means of suitable measuring instruments,

I have made the surprising discovery that the application of an adherent continuous brittle film on a material of relatively high elastic limit makes otherwise not apparent fiaws or cracks in such material capable of easy detection. I have particularly found that the application of a suitable load to an article thus coated causes the brittle film to crack preferentially above such flaws or cracks. Since cracks in the continuous brittle film are easily detectable, such cracks serve to indicate the presence, size and direction of defects in the rigid material.

It is, therefore,'an important object of this invention to provide a method of treating rigid materials of relatively high elastic limit to make cracks or flaws therein capable of easy detection.

Another important object of this invention is to provide a method of making flaws and cracks in metallic articles capable of easy visual detection by coating such articles with continuous brittle films and applying suitable loads thereto.

Other and further important objects of this invention will become apparent from the appended description and claims.

The present invention is based on the discovery that superficial defects in rigid materials of relatively high elastic limit effect a concentration above or around themselves of strains not parallel to the direction of such defects. The latter may actually penetrate the surface of the rigid material or they may exist adjacent to the surface. These defects may be rendered capable of easy detection by the following method. An article to be tested for possible flaws is coated with a continuous adherent film which is brittle enough to crack at a strain produced by a stress below the elastic limit of the material composing the article being tested. A load capable of setting up stresses not parallel to the direction of the defect is applied on the article. The magnitude of the load must be such that the stresses effected in the brittle film do not approach closely to or exceed the rupture limit of the film, over areas of sound material within the region being tested. On the other hand, the load must be large enough to effect strains which, wherever concentrated by the presence of a defect, set up stresses in the film sufficiently large to exceed the rupture limit of the film. The result is the preferential cracking of the film above the defects of the rigid material. This renders the defects capable of easy detection, since the cracks in the film can be made visible by one of several methods disclosed hereinbelow.

A continuous film cracking at a strain' of between 0.001 and 0.002 inch per inch of length has been found suitable. Numerous processes are adapted to the coating of the article to be treated with such a film. Various compositions such as lacquers, varnishes, resinous compositions and the likemay be applied to effect a partial or complete coating by dipping, brushing, spraying or melting on. Such a coating must be allowed sufiicient time to acquire its brittle properties.

The brittle films disclosed in my co-pending application entitled Method of determining strain concentration in rigid articles, filed simultaneously with the present application, are also suitable for the purposes of the present invention.

The stressing of the surface may be effected by subjecting the article to a suitable tensile, bending, torsional, or impact load applied either as a steady, repeated or impact loading. An impact load such as that delivered by a blow with a hammer has been found particularly suitable. For delicate articles which might be marred by a steel hammer, a wooden rubber or rawhide mallet may be used. The magnitude of the load 'required depends on the modulus of elasticity of the material being tested. Aluminum alloys require approximately one-third of the load necessary for steel. The latter metal requires a load sufilcient to set up a tensile stress normal to the direction of any defects of between 1,000 and 60,000 lbs. per square inch, depending on the strain concentration effected by the defect. Where the crack to be rendered capable of easy detection is exceedingly minute, as in the case of a grinding crack less than one thousandth of an inch deep, the stress is suitably kept within the limits of 10,000 and 40,000 lbs. per square inch.

Methods of application of an impact blow must be adjusted to the shape and size of the material and to the direction of the suspected cracks. Plates are best loaded by striking the reverse side directly behind the cracks. Transverse cracks in rods may be detected by striking the end or by blows producing bending. strains. Longitudinal cracks in rods require torsion loading produced by glancing surface blows or better by striking an arm fastened on the rod. Bulky shapes may be struck on several sides perpendicularly to the surface'being investigated or struck on the surface being investigated immediately adjacently to the suspected surface area. The force of the blow must be proportioned to the size of the object. A blow producing a good sharp ringing vibration is usually sufilcient. In terms of strain from 0.00005 to 0.0005 inch per inch tension must be applied normally to the suspected surface crack. It is possible to so heavily strain the material by an excessive impact that general strain cracks will form in the brittle film. In actual practice, however, this occurs but rarely; the resulting cracks are easily recognized and distinguished by the regular and closely spaced pattern of cracks formed.

If the adherent brittle film is transparent, and if the surface coated thereby is uniformly lightrefiecting, cracks in this brittle film are made easily visible by rays of light oblique thereto when viewed from the same angle as the impinging I light. The easy visibility of cracks in glass depends on a similar phenomenon, the rays of light being reflected from the surfaces of the crack. When an opaque brittle coating is used, staining the cracks with a suitable stain, if necessary preceded by enlarging the cracks by etching with a suitable solvent, will render cracks in the brittle film easily visible by reflected light.

If an acid resistant brittle film is used, cracks therein may be etched sufiiciently to expose the underlying metal surface, which is. then etched with a suitable acid. After removal of the remaining brittle film, the presence, size and direction of defects in the metallic surface are indicated by etched areas thereon.

It is sometimes necessary to examine articles such as castings which do not have smooth, uniformly light-reflecting surfaces. Cracks produced in a transparent continuous brittle film covering a rough surface may be hard to detect even when illuminated obliquely. This difiiculty may be overcome by giving the article a preliminary coating of a uniformly light-reflecting film on top of which the brittle film is applied. This undercoating may comprise bright metallic particles dispersed in lacquer or varnish, or any other composition of matter which is capable of producing a uniformly light-reflecting surface beneath the brittle film and which will not affect or be affected by the brittle film deposited thereon. Cracks in the overlying transparent film are easily visible when illuminated obliquely, and thereis no confusion with incidental surface markings on the article being tested.

To illustrate a few of the many specific processes embodying the principles of this invention, the following examples are given which descril': a few of the specific processes referred to in conjunction with the appended drawing, in which:

Figure 1 represents a longitudinal section of a metallic object treated according to my invention;

Figure 2 represents a longitudinal section of a similar metallic object treated according to a variant of my invention;

Figure 3 represents a front view of an airplane propeller being coated with a brittle film;

Figure 4 represents a front view of an airplane propeller coated according to my invention;

Figure 5 represents a lateral view of an airplane propeller coated according to my invention being stressed;

Figure 6 represents a lateral view of an airplane propeller being illuminated after treatment according to my invention;

Figure 7 represents .a front view of an airplane propeller wherein a defect has been rendered visible according to my invention;

Example I In Figure 1 the reference numeral l indicates a steel bar having a rectangular cross-section. This bar I0 has on its upper surface II a minute transverse surface crack I2 which is not visible to the naked eye. A rosin solution is applied, for instance, by brushing, on the bar ID, to cover the surface I l with the continuous transparent adherent film l3. The rosin solution may suitably have the following composition:

Grams Wood rosin K treated with 15% calcium acetate Carbon disulfide 8 Dibutyl-phthalate 0.4

After the film l3 has been allowed to'dry sufliciently, which requires about five minutes when the above composition is used, a load is applied on the bar sufficient to cause a longitudinal tensile strain of between 0.001 and 0.002 inch per inch of length over the defect. The direction of stresses around the defect i2 is indicated by the arrows l4. These stresses efiect strains which are concentrated adjacently to the defect l2, causing the film I 3 to rupture above and in a direction parallel to the defect l2, to form the crack IS. The crack l5 may be rendered visible by illumination oblique thereto.

The crack IS in the film l3 thus renders the presence, direction and size of the otherwise not apparent crack I! in the bar l0 easily detectable.

Another method of utilizing the cracks in the brittle film for indicating the presence of defects comprises etching the cracks with gasoline down to the steel, then etching the metal surface so exposed with a suitable acid which does not attack the brittle film and finally removing the remaining brittle film, when the presence, size and direction of defectsin the metallic surface will be indicated by etched areas thereon.

Example II In Figure 2 the reference numeral 20 indicates a superficially somewhat rough bar of cast iron having a rectangular cross-section. This bar 20 has on its upper surface 2| a minute transverse crack 22 which is not visible to the naked eye. A suitable aluminum pigmented nitro-cellulose composition is applied on the bar 20, for instance by spraying, to form a uniformly light-reflecting undercoat 23 on the surface 2|, which neither is affected by the rosin solution employed in Examle I nor affects the film deposited by the latter. The following nitro-cellulose composition has been found to meet these requirements:

with a black stain described hereinbelow. Such Grams a staining is suitably preceded by a brief wash- 20 second nitro-cellulose 1 ing with gasoline, to enlarge the cracks by etch- High-leafing aluminum powder 3 ing. The stained cracks of the opaque layer will Ethyla icetate 30 indicate any superficial defects exactly as the cracks in the transparent film.

After the undercut 23 haibeen a11wed A suitable black stain can be made up from 1 dry sufficiently which in case of the above gram carbon black, 0.01 gram sodium lauryl sulposition requires about 5 minutes, a brittle film fate and grams Water is applied abovethe nderwet 23 by a It is to be particularly noted that these deess identical with that described in Example I. Scripuve examples and the drawing comprised The bar 20 is then stressed as described in Extherein do not in any way limit the scope of my ample I, the arrows 25 indicating the direction of invention The revl 1 t t d b he stresses. The result is a crack 26 in the p (ms 150 osurel ra e y t these examples will teach those skilled in the art brittle film 23 which, whenrendered visible as how to apply the principles of my invention to desengmd in i g g g g the presence the detection of hidden defects in articles comdn'eet on and s Z9 0 9 posed of many other substances such as Bakelite, Example Wood, hard rubber, porcelain and the like, by

coating them with a film of a more brittle mate- I Figures 3 t0 7 ineluslve the reference rial and subsequently applying a suitable load to meral 39 indicates the blade of an airplane prot thus coated article I am aware t Feller fabricated from an aluminum alloy- The merous details of this process may be varied area 3| at the tip of the p op blade is tested without departing from the principles of this infor the Presence of fatigue cracks by be ng fi vention, and I, therefore, do not purpose limiting cleaned of di and Paint e Provide uniformly the patent granted hereon otherwise than neceslight-refiecting surface to which a brittle film t t by the prior t can adhere. The blade is then into a I claim as my invention: container indicated in Figure 3 by the reference The method of making u rficial flaws in numeral 32 which contai s a liquid composition a rigid article of high elastic limit capable of 33 which on drying coeling 1S capable of easy detection which comprises coating said arering the area. a Suitable adherent tj le a ontinuous adherent having a coat. For th s pu p molten reslfls Such as rupture limit substantially lower than the elastic wood rosin 01 V o syhthetie Tesmsmolten limit of said article and applying to said article sugar or inorganic composltlons such as certain a load of such magnitude and direction that the molten glasses Or glazes may be used- The resin resulting stresses will exceed the rupture limit solut on e p y n Examples 1 and I1 is also of said film only above said flaws while remainefiective- If it is desired t to remove any paint ing below the elastic limit of said rigid article. present, the undercoat disclosed in Example II The method f making perfi i l flaw in a may be app ed above the Paint and bntue rigid article capable of easy detection which film deposited on t undereoat as 111 comprises coating said article with a continuous ample II. adherent brittle film and applying to said article After the brlttle film has been allowed to dry a load of such magnitude and direction that the or 6001 suffici y, t tip of the Propeller blade resulting strain will cause said film to crack only which latter is either still mounted in an engine above said fi or held at the same spot in a jig, is struck several The treatment of a metallic article for sharp blows on both the front andrear faces with ing superficial defects therein capable of easy a rubber l The P of lmpact detection which comprises coating said article a y some dlstanee away from the Partlcmar with a continuous adherent film which will crack area being tested, Smee the eoatmg is destroyed at a strain of between about 0.001 and about at e Point Of h The Surface coated 1s 0.002 inch per inch of length, and applying to then examined with a concentrated light source said article a load of such magnitude and direc Such as the flash e 35 Show m Figure 6 tion that the resulting strain will reach a nuheld at'en to the the coated merical value of between about 0.001 and about area bei vlewed from the some angle as h 0.002 inch per inch cmength only above said impinging light rays. The presence, size and didefects rection of fatigue cracks will then be indicated The method of making superficial flaws by corresponding easily visib e Cracks Such as the a metallic article capable of easy detection which i g ui e l by the reference numeral 36 on mprises coating said article with a continuous adherent brittle lacquer film, and applying to said If an opaque bnttle layer used on the Same article an impact load of such magnitude that propeller blade, no undercoatmg is gecesisaar t to Said mm will crack only above Said, flaws Provide an ndeflyntg unifrm1.y.1ig ec mg 5. The method of making superficial flaws in a Surface The followlflg composltmn may be rigid article of high elastic limit capable of easy for this purpose' Grams 'detection which comprises coating said article with a continuous transparent adherent film havmsm K treated wlth calcium 5 ing a rupture limit substantially lower than the tete 8 elastic limit of said article, applying to said ar- Cerben dlsumde 0 6 ticle a load of such magnitude and direction that i i -o l the resulting stresses will exceed the rupture limit Tltemum dloxide of said film only above said flaws while remain- This composition requires 12 hours to dry n ing below the elastic limit of said rigid article, factorfly and obliquely illumnating said film whereby the The propeller blade is Stressed as described presence, size and direction of said fiaws' may be hereinbefore. Any cracks in the opaque brittle noted fr m the P nce, size and direction of layer can be made easily visible by being stained y crac s n said film.

6. The method of making superficial flaws in,

a rigid article capable of easy detection which comprises coating said article with a transparent continuous adherent brittle film, applying to said article a load of such magnitude and direction that the resulting strain will cause said film to crack only above said flaws, and obliquely illuminating said film whereby the presence, size and direction of said flaws may be noted from the presence, size and direction of any cracks in said film.

7. The treatment of metallic articles for malt--v ing superficial defects therein capable of easy detection which comprises coating said article with a continuous adherent transparent film which will crack at a strain of between 0.001 and about 0.002 inch per inchot length, applying to said article a load of such magnitude and direction that the resulting strain will reach a numerical value of between about 0.001 and about 0.002 inch per inch of length only above said defects, and obliquely illuminating said film whereby the presence, size and direction of said flaws may be noted from the presence, size and direction of any cracks in said film;

8. The method of making superficial flaws in a metallic article capable of easy detection which comprises coating said article with a transparent continuous adherent brittle lacquer film, applying to said article an impact load of such magnitude that said film will crack-only above said flaws, and obliquely illuminating said film whereby the presence, size and direction of said flaws may be noted from the presence, size and direction of any cracks in said film.

9. The-methodof making superficial flaws in a rigid article of high elastic limit capable of easy detection which comprises coating said article with a continuous adherent opaque film having a rupture limit substantially lower than the elastic limit of said article, applying to said article a load of such magnitude and direction that the resu'ting stresses will exceed the rupture limit of said film only above said flaws while remaining below the elastic limit of said rigid article, and treating said film with a composition suitable for staining any cracks therein, whereby the presence, size and direction of said flaws may be noted from the presence, size and direction of stained cracks in said film.

10. The method of making superficial flaws in a rigid article of high elastic limit capable of easy detection which comprises coating said article with a continuous opaque adherent film having a rupture limit substantially lower than the elastic limit of said article, applying to said article a load of such magnitude and direction that the resulting stresses will exceed the rupture limit of said film only above said flaws while remaining below the elastic limit of said rigid article, etching said film with a suitable solvent and treating said film with a composition suitable for staining any cracks therein whereby the presence, size and direction of said flaws may be noted from the presence, size and direction of stained cracks in said film.

11. The method of making superficial flaws in a rigid article of high elastic limit capable of easy detection which comprises coating said article with a continuous adherent film having a rupture limit substantially lower than the elastic limit of said article, applying to said article a load of such magnitude and direction that the resulting stresses will exceed the rupture limit of said film only above said flaws while remaining below the elastic limit of said rigid article, etching said film with a suitable solvent so as expose the surface of said article solely below any cracks in said film, etching said exposed surface with a. substance to which said film is re-' sistant, and removing said filmwhereby the pres-. i ence, size and direction of said flaws may be noted from the presence, size and direction of etched areas on said surface of said article.

12. The method of making superficial flaws in a metallic article capable of easy detection which I comprises coating said article with an opaque continuous adherent brittle film and applying to said article an impact load of such magnitude that said film will crack only above said flaws.

13. The method of making superficial flaws in 5 a metallic article capable of easy detection which comprises coating said article with an opaque continuous adherent brittle film, applying to said article an impact load of such magnitude that said film will crack only above said flaws and 1 treating said film with a composition suitable for staining any cracks in said fihn whereby the presence, size and direction of said flaws may be noted from the. presence, size and direction of stained cracks in said film. 1

14.. The method of making superficial flaws in metallic articles having surfaces which do not refiect light uniformly capable of easy detection which comprises coating said surfaces with an adherent film capable'of-producing a uniformi iy light-reflecting surface beneath a continuous adherent transparent brittle film, coating said light-reflecting film with a continuoustransparent adherent brittle film and applying to said article an impact load of such magnitude that 1 said brittle film will crack only above said fiawal 15. The method of making superficial fiaws ini metallic articles having surfaces which do not? reflect light uniformly capable of easy detection? which comprises coating said surfaces with an 1 adherent film capable of producing a uniformly light-reflecting surface beneath a continuous adherent transparent brittle film, coating said light-reflecting film= with a continuous transparent adherent brittle film, applying to said article an impact load of such magnitude that said brittle film will crack only above said flaws, and obliquely illuminating said brittle film whereby the presence, size and direction of said flaws may be noted from the presence, size and direction I of any-cracks in said brittle film.

16. The method of making superficial fiaws in a metallic article having surfaces covered with an adherent film capable of easy detection which comprises coating said surfaces with an opaque continuous adherent brittle film, applying to-said article an impact load of such magnitude that said brittle film will crack only above said fiaws and treating said brittle film with a'. composition suitable for staining any cracks in said brittle: film whereby the presence, size and direction of} said flaws may be noted from the presence, sizef and direction of stained cracks in said brittle; film.

GREER. ELLIS. '4 

